The present invention relates to a process and an apparatus for the controlled pyrolysis of plastic materials.
The growing demand for clean energy and the need to dispose waste which degrades naturally only with difficulty, such as plastics from hospital, municipal and industrial waste, is radically changing the problem of disposal, which can no longer be considered as the more or less uncontrolled burial used so far.
On the other hand, thermodestruction technologies are unsuitable and dangerous due to the fumes generated by combustion. Various pyrolysis plants have been started in recent years in an attempt to recover energy-yielding products from plastics; all these plants tend to convert the polymer, by heat emission, into a plurality of products to be used both in combustion to generate electric power and in order to eliminate the acid residues that produce toxic fumes.
The technologies currently used in this field employ high temperatures (500-800xc2x0 C.) in static and/or rotary furnaces with very long contact times (3-4 min); this entails very high plant costs without an appropriate economic efficiency.
Various disadvantages are observed:
1. direct combustion entails the forming of carcinogenic products, such as dioxins and furfurans;
2. any attempts to directly bum the gasification products instead of the residue of the sorting of municipal solid waste (dry fraction constituted by paper, rags and plastics) does not solve the problem of the emission of the above-mentioned carcinogenic fumes, and gasification contributes to an increase in operating costs;
3. the presence of plastics such as PVC leads to high corrosion of the combustion equipment, and it is impossible to use high-temperature boilers due to he inevitable corrosion of their tube nest.
Thermal cracking and catalytic cracking have long been used in refineries for the pyrolysis of organic components having a medium relative molecular mass. The thermodynamic mechanisms have been described in many documents, including Gates, B. C. et al., Chemistry of Catalytic Processes, McGraw-Hill, Inc. (1979) and Pines, H., The Chemistry of Catalytic Hydrocarbon Conversions, Academic Press (1981).
The principal aim of the invention is the elimination of environmental impact problems, recovery of heat energy and minimal production of screening reject. This aim is achieved by a process and an apparatus organized according to the pyrolysis (cracking) process, of the present invention.